A diffractive optical element which has a diffraction grating provided on a lens body and which effects convergence or divergence of light by utilizing a diffraction phenomenon is called a diffraction grating lens. It is widely known that a diffraction grating lens is good for correcting for lens aberrations such as curvature of field or chromatic aberration (a shift in an image point depending on wavelength). This is because a diffraction grating may have an opposite dispersion (inverse dispersion) to a dispersion which is caused by the optical material, or have a dispersion (anomalous dispersion) deviating from the linearity of dispersion of the optical material. Therefore, when combined with usual optical elements, a diffraction grating lens exhibits a great ability of correcting for chromatic aberration.
Moreover, when a diffraction grating is used for an optical system for imaging purposes, the same performance can be attained with fewer lenses than is possible with an optical system for imaging purposes that is composed only of aspherical lenses. This provides an advantage in that the production cost for the optical system for imaging purposes can be reduced and that the optical length can be shortened, thus realizing a low profile.
With reference to FIGS. 21(a) to (c), a conventional method for designing the shape of a diffraction grating lens will be described. A diffraction grating lens is mainly designed by a phase function method or a high-refractive-index method. Herein, a designing method based on the phase function method will be described. The end result will also be the same when the high-refractive-index method is used for designing.
The shape of a diffraction grating lens is formed from the base shape of a lens body on which the diffraction grating is provided, as well as from the shape of the diffraction grating. FIG. 21(a) shows an example in the case where the surface configuration of the lens body is an aspherical shape Sb, whereas FIG. 21(b) shows an example shape Sp1 of the diffraction grating. The diffraction grating shape Sp1 shown in FIG. 21(b) is determined by a phase function. The phase function is expressed by eq. (1) below.
                              [                      math            .                                                  ⁢            1                    ]                ⁢                                                                                                ϕ          ⁡                      (            r            )                          =                                            2              ⁢              π                                      λ              0                                ⁢                      ψ            ⁡                          (              r              )                                                          (        1        )                                          ψ          ⁡                      (            r            )                          =                                            a              1                        ⁢            r                    +                                    a              2                        ⁢                          r              2                                +                                    a              3                        ⁢                          r              3                                +                                    a              4                        ⁢                          r              4                                +                                    a              5                        ⁢                          r              5                                +                                    a              6                        ⁢                          r              6                                +          …          +                                    a              i                        ⁢                          r              i                                                                                          (                              r            2                    =                                    x              2                        +                          y              2                                      )                                        Herein, φ(r) is a phase function; Ψ(r) is an optical path difference function; r is a distance from the optical axis along a radial direction; λ0 is a design wavelength; and a1, a2, a3, a4, a5, a6, . . . , ai are coefficients.
In the case of a diffraction grating which utilizes first-order diffracted light, an annular zone is provided at every point where the phase from a reference point (center) reaches 2π in the phase function φ(r), as shown in FIG. 21(b). The shape Sbp1 of the diffraction grating plane shown in FIG. 21(c) is determined by adding the shape Sp1, which is based on the curve of the phase difference function being cut up every 2π, to the aspherical shape Sb of FIG. 21(a). Specifically, the value of the phase function of FIG. 21(b) is translated into an optical path length so that the step surface height of each annular zone equals d that satisfies eq. (2) below, and this is added to the surface configuration Sb of the lens body shown in FIG. 21(a).
                              [                      math            .                                                  ⁢            2                    ]                ⁢                                                                                      d        =                              m            ·            λ                                                              n                1                            ⁡                              (                λ                )                                      -            1                                              (        2        )            Herein, m is a design order (m=1 in the case of first-order diffracted light); λ is a wavelength used; d is a step surface height of the diffraction grating; and n1(λ) is the refractive index of a lens material which composes the lens body at the used wavelength λ. The refractive index of the lens material has wavelength dependence, and is a function of wavelength. In any diffraction grating that satisfies eq. (2), the phase difference at steps between annular zones is 2π, and the diffraction efficiency of first-order diffracted light relative to light of the used wavelength (hereinafter referred to as “first-order diffraction efficiency”) can be made approximately 100%. According to eq. (2), when the wavelength λ changes, the value of d that makes the diffraction efficiency 100% will also change. Conversely, if the d value is fixed, the diffraction efficiency will not be 100% at any wavelength other than the wavelength λ that satisfies eq. (2).
However, in the case where a diffraction grating lens is used for generic imaging purposes, there is a need to diffract light in a broad wavelength band (e.g., a visible light region spanning wavelengths of about 400 nm to 700 nm). Consequently, as shown in FIG. 22, when a visible light beam 223 enters a diffraction grating lens having a diffraction grating 222 provided on a lens body 221, not only first-order diffracted light 225 which is ascribable to light of the wavelength that is selected as the used wavelength λ, but also diffracted light 226 of orders that are unwanted (hereinafter also referred to as “diffracted light of unwanted orders”) occurs. For example, if the wavelength which determines the step surface height d is a wavelength of green (e.g., 540 nm), then the first-order diffraction efficiency at the green wavelength will be 100%, so that no diffracted light 226 of unwanted orders will occur at the green wavelength; however, the first-order diffraction efficiency will not be 100% at a red wavelength (e.g., 640 nm) or a blue wavelength (e.g., 440 nm), so that 0th order diffracted light of red or second-order diffracted light of blue will occur. These 0th order diffracted light of red and second-order diffracted light of blue are the diffracted light 226 of unwanted orders, which will spread across the image plane in the form of a flare or ghost, thus deteriorating the image or degrading the modulation transfer function (MTF) characteristics.
As shown in FIG. 23, Patent Document 1 discloses providing an optical adjustment film 231 which is composed of an optical material having a different refractive index and a different refractive index dispersion from those of the lens body, on the surface of a lens body 221 having a diffraction grating 222 formed thereon. Patent Document 1 discloses that, by prescribing specific conditions for the refractive index of the lens body 221 having the diffraction grating 222 formed thereon and the refractive index of the optical adjustment film 231 formed so as to cover the diffraction grating 222, it is possible to reduce the wavelength dependence of diffraction efficiency, reduce diffracted light of unwanted orders, and suppress flare due to diffracted light of unwanted orders.
Patent Document 2 discloses, in order to prevent reflected light from step surfaces 232 of a diffraction grating from becoming flare light as it is transmitted through a blazed surface, providing light absorbing portions near the feet of the slopes of annular zones to allow reflected light from the step surfaces to be shaded by the light absorbing portions.